Battery cells such as flooded lead-acid, absorbed-glass-matt (AGM), lead-acid, Nickel Cadmium (NiCd), Nickel Metal Hydride (NiMh) and the like perform best at certain temperature ranges and are easily damaged when exposed to very high temperatures. When such battery cells, either standalone or within a battery pack, are exposed to certain high temperatures, various physical changes occur internal to the battery cells such as boiling of the electrolyte, etc. In an extreme case, such as boiling of the electrolyte, high pressure results within the sealed cell, leading to possible deformation of the outer case, deformation of the anode/cathode arrangements and, possible out-gassing or leakage of electrolyte, the later of which resulting in a totally useless cell.
Many hospital or surgical related devices use battery packs, providing enhanced range of motion to surgeons and reducing the chance of a power cord finding its way into a bad location. There are many examples of such devices for drilling into bone, sawing bone, screwdrivers, making incisions, etc. These devices use battery packs that attach to the devices and provide power during, for example, an operation such as an orthopedic operation. In most systems, the battery pack is removable for charging in a charging station.
A recent search indicates that many battery packs for such devices contain the heavy metal cadmium, Cd. The labels of these batteries show the chemical symbol, Cd, along with a crossed-out trash can, meaning that these batteries are not to be disposed of in ordinary trash due to land, water table and air pollution from landfills or incinerators. Notwithstanding, the cost for such battery packs are around $150-$400 for a used pack and $300-$500 for a new pack.
Some battery packs for medical devices are single-use, in that after usage, the entire pack is discarded. This creates a dilemma because the battery packs are often exposed to body fluids, making them a bio-hazard. Bio-hazard material is often incinerated to neutralize the hazard, but most batteries cannot be incinerated due to pollution and/or explosion issued. Therefore, the battery packs should not be placed in bio-hazard disposal containers, yet, since they are now bio-hazardous, the exposed battery packs cannot be disposed in normal recycle locations.
Many battery packs are rechargeable and reusable, much like a battery pack for a home cordless drill. After use and before the next usage, the device and the battery packs must be sterilized to kill any pathogens present on surfaces and in cracks, etc. To sterilize a rechargeable battery pack, per one exemplary manufacturer's procedure, the device is separated from the battery pack and any debris or accumulation is cleaned. Next, sterilization is performed through an Autoclave Cycle. Autoclave cycles are, for example, 132° C.-137° C. for at least 15 minutes then 5 minutes drying time or 15 minutes of pre-heating, 132° C.-137° C. for at least 10 minutes, then 5 minutes drying time. Such cycles, although hard on mechanical devices such as motors, usually do not severely affect the life of the actual devices. Unfortunately, these heat cycles often severely affect the life of the battery packs. For example, nickel cadmium battery packs are known to severely degrade after such sterilization cycles. A battery pack that normally would function well after 200 charge/use cycles (at normal ambient temperatures) is only useful for around six charge/use cycles after being sterilized at such temperatures and periods of time. Furthermore, even though it is usable for six cycles, the amount of charge such a battery pack will hold after heat sterilization is severely decreased, often requiring swapping of battery packs during an operation.
Newer, ecology minded technologies such as lithium ion (Li-ion) and Lithium Ion (Li fe) normally provide more use/charge cycles than nickel cadmium, but are even more susceptible to issues related to high temperatures. In, for example, Lithium Ion battery cells, the thin Solid Electrolyte Interface (SEI) layer on the anode breaks down due to overheating caused by excessive currents, overcharging or high temperatures. The breakdown of the SEI layer starts to occur at the relatively low temperature of 80° C. Once the SEI layer is breached, the electrolyte reacts with the carbon anode at a higher, uncontrolled, temperature, creating an exothermal reaction which drives the temperature up still further. Therefore, it is important to assure that the core temperature of Lithium Ion cells remains well under 80° C., preferably under 75° C.
It is recommended to store nickel cadmium battery cells at less than 45° C. and temperatures above this “can” cause the alkaline electrolyte to leak out. Storage of nickel cadmium, lithium-Fe—S and Lithium-Mg—O at temperatures above 60° C. violates most manufacturers' recommendations and many warranties.
Unfortunately, when heated in an Autoclave to a temperature between 132° C. and 137° C., most battery cells will fail. Because each cell has an initial temperature and mass and the case, usually ABS plastic, has minor insulating ability, exposure to a temperature between 132° C. and 137° C. does not mean that, after 10, 20 or 30 minutes of exposure, the internal temperature will reach between 132° C. and 137° C., but it will likely be well over 80° C., thereby permanently damaging, for example, lithium ion battery cells.
What is needed is a battery pack that is suitable for sterilization in an Autoclave at sterilization temperatures for long enough periods of time as to properly sterilize the battery pack while preventing damage to the internal battery cell(s).